The peroxidase immunocytochemical technique has been adopted for the localization of apolipoproteins in human tissue. Antibodies to apolipoproteins C-I, C-II, C-III, B, D, A-I, H, and E have been developed in the Branch. Several tissues have been obtained for analysis of apolipoprotein content. Human jejunen from Tangier patients was shown to contain apoA-I, apoA-II, and apoB. These results were consistent with data from metabolic studies which indicated that patients with Tangier disease were able to synthesize apoA-I, apoA-II, and apoB. The defect in this disease is therefore not due to defective synthesis of the apolipoproteins. The coronary atherosclerotic lesions of patients with Type III hyperlipidemia was investigated and examined by the peroxidase technique. These lesions were similar to those observed in "typical" atherosclerotic lesions. Peroxidase staining revealed that the foam cells were positive for apolipoproteins A-I, A-II, B, C-I, and C-II. These studies have indicated that apolipoproteins associated with both LDL and HDL are present within the foam cells.